Recently, techniques have been proposed for efficiently compressing digital audio-video programs for storage and transmission. See, for example, ISO.backslash.IEC IS 13818-1,2,3: Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Systems, Video and Audio ("MPEG-2"); ISO.backslash.IEC IS 11172-1,2,3: Information Technology-Generic Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbits/sec: Systems, Video and Audio ("MPEG-1"); Dolby AC-3; Motion JPEG, etc. Herein, the term program means a collection of related audio-video signals having a common time base and intended for synchronized presentation, as per MPEG-2 parlance.
MPEG-1 and MPEG-2 provide for hierarchically layered streams. That is, an audio-video program is composed of one or more coded bit streams or "elementary streams" ("ES") such as an encoded video ES, and encoded audio ES, a second language encoded audio ES, a closed caption text ES, etc. Each ES, in particular, each of the audio and video ESs, is separately encoded. The encoded ESs are then combined into a systems layer stream such as a program stream "PS" or a transport stream "TS". The purpose of the PS or TS is to enable extraction of the encoded ESs of a program, separation and separate decoding of each ES and synchronized presentation of the decoded ESs. The TS or PS may be encapsulated in an even higher channel layer or storage format which provides for forward error correction.